An on-load tap changer of this type along with an associated force-storing unit is known from publication of the applicant “On-load tap Changer VACUTAP® VV Operating Instructions,” imprint BA164/03de. This on-load tap changer combines the function of a selector for the no-load preselection of the tap to which the changeover is to be made, as well as a load transfer switch for the actual switching.
The force-storing unit of this known on-load tap changer has a flexible sheet-metal plate on which several tension springs are attached at one end, the springs being pivoted at their other ends on bearings. The free end of the flexible sheet-metal plate in turn is rotatably mounted on a drive crank. The drive crank that is mounted on an operating shaft is actuated through a one-way clutch means by a drive element that is coaxial with and rotatable on and independently of this shaft. At its bottom end the operating shaft supports a Geneva crank of a Geneva drive. Finally, the Geneva wheel of this Geneva drive is connected to a rotatable switching column supported coaxially inside an insulating tube, this column supporting the switching elements on a radially projecting arm.
Fixed tap contacts are provided in the wall of the insulating tube at vertically spaced horizontal planes, these contacts being selectably connectable in each plane to one of the switching elements on the switching column.
The functional principle of this known force-storing unit is as follows:
At the start of a load-switching operation, i.e. switching from one winding tap to an adjacent different winding tap, the drive element, actuated generally by a drive motor, begins to turn slowly and continuously. The drive element entrains the drive crank via an abutment, and with it also the end of the flexible sheet-metal plate attached thereto, thereby moving and tensioning the tension springs. After an initial rotation angle of 180° when the tension springs have reached their maximum displacement, the drive element disengages from the drive crank due to the one-way clutch. After passing dead center, the next rotation of the drive crank occurs by snap action since the tension springs are still tensioned at this point in time.
This rapid rotational motion of the drive crank is transferred to the Geneva crank of the Geneva drive, and thus to the switching column; the switching elements attached thereto switch by snap action to each of the adjacent fixed tapping contacts. The tap changing operation has concluded.
This force-storing unit has proven successful over many years for the widest variety of load tap changers. Nevertheless, the tension springs used do not have unlimited mechanical fatigue strength but must be replaced after a predetermined operating life or permissible number of operations for reason of safety. This has not been a problem up to now since generally inspections are performed on the on-load tap changer in any case after this permissible number of operations. Due to the high reliability of the vacuum-type switching cells now available and used, and the overall advantages of vacuum switching technology, it is now possible, however, to increase the maintenance intervals for on-load tap changers such that the limited mechanical fatigue strength of the employed tension springs has increasingly proven to be a disadvantage for the known force-storing units.